A lithium secondary battery including a negative electrode using a carbon material such as graphite as a negative electrode active material, a positive electrode using a lithium-containing compound such as cobalt oxide, and a lithium salt such as LiPF6 as an electrolyte interposed between those positive and negative electrodes has been conventionally known. In the lithium secondary battery, lithium ions are absorbed in a negative electrode active material during battery charge, and lithium ions are discharged from the negative electrode active material during battery discharge.
In recent years, a lithium secondary battery of this type draws attention as a power source of an electric vehicle. However, graphite, which is a negative electrode active material currently widely used, has theoretical capacity of only 372 mAh/g, and higher capacity has been desired. For this reason, recently metal materials such as Si and Sn that can be expected to have high capacity have been actively studied as an alternative material of a carbonaceous negative electrode active material.
However, Si and Sn cause large volume expansion and contraction with absorption and release of lithium ions. The expansion and contraction stresses gave rise a problem that particles of Si and Sn break or separate from a collector, and cycle characteristics that are capacity retention characteristics when repeating charge-discharge are deteriorated.
As the countermeasure, Patent Document 1 describes that a negative electrode active material having a structure that a lot of Si nuclei are surrounded by an Al—Co based alloy matrix can relax expansion-contraction stress, thereby improving the cycle characteristics.
Patent Document 1: JP-A-2009-32644
However, the conventional technology had room for further improvement in the following points. That is, in the negative electrode active material having a structure that Si nuclei are surrounded by the Al—Co based alloy matrix, improvement in the cycle characteristics is observed, but it was difficult to enhance the utilization rate of an active material.
An Al alloy has a few Li activity, but it cannot say that the Al alloy sufficiently plays a function as an Li diffusion path, and the utilization rate of an active material to the theoretical capacity thereof (=initial discharged capacity/theoretical capacity of active material×100) is decreased. The phenomenon of this type may occur even in the case that Cu is selected as a matrix.